Hi,
You can use Charts Desktop or Charts Cloud to create, edit and export into PMDG rte format plans.
You can also use SimBrief to do similarly and SimBrief Downloader to have them ready to load into PMDG 737 FMC.
Please see PMDG Problem - #7 by pmdg08 and FAQ - Exporting SimBrief flight plans to PMDG 737 for MSFS
Cheers
Ian